The present invention relates to a magnetic device having two static magnetic field generating sources disposed so as to oppose each other and to a nuclear magnetic resonance imaging device (MRI) using the same.
FIG. 10 shows an example of super conducting magnet device used largely in conventional MRI devices.
A super conducting magnet device 171 which constitutes an MRI device includes a cylindrical hollow portion 174 near the center of which a uniform static magnetic field is generated. The super conducting magnet device 171 is constituted by divided solenoid coils wound around respective cylindrical bobbins. The coils are usually formed by super conducting wire material using NbNi, and function as super conducting coils carrying super conducting current under a condition being cooled at a temperature of about 4.2 k by liquid helium. The liquid helium is stored in a low temperature vessel not illustrated.
A patient representing an inspection body 172 is carried into the uniform magnetic field region existing within the cylindrical hollow portion 174 with a movable bed 173.
However, in a conventional MRI device using the magnet device with the above explained structure, since the patient had to enter into the narrow and small cylindrical hollow portion 174, the patient was subjected to an intense claustrophobia. For this reason, patients sometimes refused to enter into the device.
Further, it was difficult for an operator such as a medical doctor to access a person to be inspected during image taking so that a so called IVR (Interventional Radiology) was impossible.
As a technology for resolving the above problem, an open type MRI has been proposed recently as disclosed, for example, in JP-A-9-190913(1997).
The open type MRI device includes, for example, an upper magnet assembly 33 and a lower magnet assembly 34 and is constituted thereby as illustrated in FIG. 11.
In the drawing, super conducting coils 44 are disposed in a low temperature vessel 46 and which is contained in a vacuum vessel 45. The lower magnetic assembly is structured in the same manner.
In this open type MRI device a uniform static magnetic field in vertical direction is generated at a center portion of the magnet, in that on a geometrical center plane between both magnet assemblies which locates at equal distance from both magnet assemblies, and a patient 36 is carried through a movable bed 35 so that a portion to be imaged enters into a uniform magnetic field region 42.
With such open type MRI device, the patient 36 scarcely feels claustrophobia, because the patient 36 is entered into a highly open space.
However, in such conventional open type MRI device since a uniform magnetic field is generated in the region 42 which locates at equal distance from the upper and lower magnet assemblies as has been explained above, it is necessary to carry the patient to a position which locates at equal distance from the upper and lower magnet assemblies, when MRI images are taken.
For this reason, there was a problem that since a space portion 43 underside the patient operates as a dead space, a space 41 between the magnet assemblies is not effectively used. Further, in the drawing numeral 37 is an operator, numerals 38 and 39 are leakage magnetic field suppressing members belonging to the upper magnet assembly 33 and numeral 40 is a support column of magnetic material which connects and supports the upper leakage magnetic field suppressing members and like lower leakage magnetic field suppressing members.
In view of the above problems, an object of the present invention is to provide a magnet device for an MRI device and an MRI device using the same which removes a dead space between a lower magnet assembly and a bed, permits an effective use of a space between upper and lower magnet assemblies without increasing the size of the device and completing the structure thereof, and creates a broad upper space which provides a highly open space and permits for an operator to easily access a patient.
One aspect of the present invention for achieving the above object is to provide a magnet device which is characterized, in that the magnet device comprises, two static magnetic field generating sources which are disposed so as to oppose each other while placing an inspection body therebetween;
a first vessel which contains one of the two static magnetic field generating sources; and
a second vessel which contains the other of the two static magnetic field generating sources, wherein the first vessel and the second vessel are disposed at positions asymmetric with respect the inspection body.
Another aspect of the present invention for achieving the above object is to provide a static magnetic field generating device for an open type MRI device which is characterized, in that the static magnetic field generating device comprises a pair of magnet assemblies which are disposed in vertical direction opposing each other while sandwiching a space for receiving a person to be inspected and respectively generate magnetic fields in vertical direction, wherein an absolute value of total magnetic moment produced by an upper magnet assembly is set larger than an absolute value of total magnetic moment produced by a lower magnet assembly in the pair of magnet assemblies so that a uniform magnetic field region produced by the pair of magnet assemblies covers a position of the person to be inspected who is laid on a lower position than a geometrical center plane of the space formed by the pair of magnet assemblies.